Cationic low-coordinated pi-bonded phosphorus compounds add Lewis donors, such as amines or phosphines. In contrast to the trigonal bipyramide formed in donor-addition to the siliconium cation, the phosphorus cations add donors in a perpendicular fashion, depending on the nature of the pi-bonds toward phosphorus. According to quantum chemical calculations the various cations reveal different stabilities and hence a strong variation in donor addition abilities.